Transmission line transformers have been used at RF frequencies for many years to give broadband performance. Because the energy is coupled by a transverse transmission line mode rather than by flux linkages, as in the conventional transformers, the stray inductances and interwinding capacitances are absorbed into the characteristic impedance of the transmission line. Therefore, the response of the transmission line transformer is limited by the deviation of the characteristic impedance from the optimum value, the unabsorbed parasitics, and the length of the transmission line. The transformer does not operate as a transformer when a transmission line is a half wave length long.
Essentially the transmission lines operate in many different modes as when two transmission lines are physically close together the electromagnetic field on one affects the other so that energy can be coupled from one line to another. The coupled lines are assumed to support only transverse electromagnetic (TEM) waves so that a DC capacitance equivalent circuit can model the coupled transmission line circuit. Basically, a pair of coupled transmission lines with a common ground exhibit an equivalent circuit which includes ideal transformers. These structures are well known. See for example a text entitled "Microwave Semiconductors Circuit Design" by W. Allan Davis published by Van Nostrand Reinhold Company, 1984, Chapter 3 entitled "Impedance Transformers and Filters".
At RF frequencies (1 MHz to 100 MHz) transmission line transformers are constructed by coiling the transmission line on a ferrite core so that undesired modes are inhibited. Typically a transformer of this type could be made having a bandwidth of several decades in frequency. These types of transformers also have been constructed at microwave frequencies using a microstrip or planar approach. See an article entitled "Analysis of Rectangular Spiral Transformers for MMIC Applications" by A. Boulouard and M. E. Le Rouzic in "IEEE Transactions on Microwave Theory and Techniques", Vol. 37, No. 8, August 1989. The trifilar transformer described in that article is typical of the prior art as it consists of three parallel coupled lines wrapped in a spiral in a planar surface. The resulting transformer gives slightly better than an octave bandwidth.
Such transformers have problems which result from the physical configuration of the transformer. Certain of the problems exist because the outside and inside lines have a different characteristic impedance than the center line. Normally the outside line is longer than the inside line and the even and odd mode phase velocities travel at different speeds thus further creating problems with the designs.
The prior art devices were used in microwave monolithic integrated circuits (MMICs) on gallium arsenide (GaAs) substrates.
The use of such transformers on such substrates employing microstrip or planar techniques is extremely desirable and it is an object of the present invention to provide a microstrip transformer apparatus which can be fabricated on a planar surface using either microstrip or other planar technology.